Field of the Invention
The present invention generally relates to the field of induction heating. More specifically, the present invention relates to inverters for induction heating apparatuses.
Overview of the Related Art
Induction heating is a well-known method for heating an electrically conducting load by inducing eddy currents in the load through a time-varying magnetic field generated by an alternating current (hereinafter, simply AC current) flowing in an induction heating coil. The internal resistance of the load causes the induced eddy currents to generate heat in the load itself.
Induction heating is used in several applications, such as in the induction cooking field, wherein induction heating coils are located under a cooking hob surface for heating cooking pans made (or including portions) of electrically ferromagnetic material placed on the cooking hob surface, or in the ironing field, wherein induction heating coils are located under the main surface of an ironing board for heating an electrically conducting plate of a iron configured to transfer heat to clothes when the iron travels over the ironing board (similar considerations apply to a pressure iron system).
The amount of heat generated in the load depends on the electric power delivered to the load through the induction heating coil, which in turn depends on the frequency of the AC current flowing through the latter, the coupling between the load and the induction heating coil, and the time spent by the load at the induction heating coil.
Usually, the AC current used to generate the time-varying magnetic field is generated by means of an inverter circuit, such as a half bridge inverter, a full bridge inverter, or a quasi-resonant inverter, comprising a switching section including power switching elements, such as for example Insulated-Gate Bipolar Transistors (IGBT), and a resonant section comprising inductor(s) and capacitor(s), with the induction heating coil that is an inductor of the latter section. The inverter circuit is configured to receive an input alternating voltage (hereinafter, simply AC voltage), such as the mains voltage taken from the power grid, and to accordingly generate an AC current (flowing through the induction heating coil) oscillating at a frequency corresponding to actuation frequency of the power switching elements (i.e., the frequency with which they are switched between the on and the off state) and having an envelope following the input AC voltage, with the amplitude of the envelope that depends in turn on the actuation frequency itself (the lower the actuation frequency, the higher the amplitude thereof). The current flowing through the induction heating coil is sourced/drained by the power switching elements of the switching section.
Taking into consideration the half bridge architecture, in order to correctly operate the power switching elements in safe conditions, the actuation frequency should be kept lower than a maximum frequency depending on the type of power switching elements. For example, for standard IGBTs, such maximum frequency may correspond to 50-60 kHz.
As already mentioned above, the electric power delivered to the load through the induction heating coil depends on the frequency of the AC current flowing through the latter. With an inverter circuit of the type described above, the electric power provided to the load is at its maximum when the current flowing through the induction heating coil oscillates at the resonance frequency of the resonant section, i.e., when the actuation frequency is equal to the resonance frequency.
As it is well known to those skilled in the art, if the power switching elements are driven for a certain time at actuation frequencies lower than resonance frequency, the power switching elements may be irreparably damaged because of heat dissipation, and control instability due to loss of soft switching conditions.
Therefore, to ensure safe actuation of the inverter circuit, the actuation frequency should be set to be:                lower than the power switching element maximum frequency;        higher than the resonance frequency.        
While the first value is fixed and known in advance (depending on the type of power switching elements), the resonance frequency strongly depends on the coupling between the induction heating coil and the load, i.e., it depends from a series of unpredictable features such as the type of load, the distance between load and induction heating coil, the geometry of the load and of the induction heating coil.
Devices which exploit induction heating should be provided with a control unit specifically designed to avoid that the actuation frequency falls outside the safe range defined above. When a user of a device of this kind is requesting a specific electric power (e.g., corresponding to a specific temperature to be reached by a cooking pan or by a clothes iron), such control unit has to check whether the desired electric power requested by the user corresponds to an actuation frequency which falls within the safe range. In the affirmative case, the control unit is configured to dispense the requested electric power. In the negative case, the exact request of the user cannot be satisfied, and the control unit may be configured to set the electric power to a safe level different to the requested one.
In order to ensure safe actuation of the inverter circuit, a further constraint has to be fulfilled, relating to the maximum current that the power switching elements are able to sustain for a certain time without damage. For example, standard IGBTs, commonly used in household appliances for induction applications, are designed to sustain current values not higher than 50-60 A.
For this reason, the inverter circuit is usually provided with a clamping circuit configured to clamp the current flowing through the induction heating coil before it reaches the maximum current that can be sustained by the power switching elements. Moreover, the inverter circuit is further provided with a software protection configured to clamp the actuation frequency if said maximum current is approached, before the activation of the clamping circuit for the current.
Since the envelope of the AC current flowing through the induction heating coil has an amplitude that depends on the actuation frequency (the lower the actuation frequency, the higher the amplitude thereof), it is not possible to known a priori whether a selected actuation frequency corresponds to a current flowing through the induction heating coil that is lower than the maximum current or not.
EP1734789 discloses a method involving providing an alternating supply voltage and a frequency converter with an adjustable switching unit. The operating frequency of the switching unit and/or the frequency converter is increased from a frequency base in the course of half cycle of the voltage. The frequency is then decreased to the base, so that the frequency amounts to the base, at the zero crossing of the supply voltage.